Modern automobile engines require high octane gasoline for efficient operation. Previously, lead and oxygenates, such as methyl-t-butyl ether (MTBE), were added to gasoline to increase the octane number. Furthermore, several high octane components normally present in gasoline, such as benzene, aromatics, and olefins, must now be reduced. Obviously, a process for increasing the octane of gasoline without the addition of toxic or environmentally adverse substances would be highly desirable.
Gasoline is generally prepared from a number of blend streams, including light naphthas, full range naphthas, heavier naphtha fractions, and heavy gasoline fractions. The gasoline pool typically includes butanes, light straight run, isomerate, FCC cracked products, hydrocracked naphtha, coker gasoline, alkylate, reformate, added ethers, etc. Of these, gasoline blend stocks from the FCC, the reformer and the alkylation unit account for a major portion of the gasoline pool.
For a given carbon number of a light naphtha component, the shortest, most branched isomer tends to have the highest octane number. For example, the singly and doubly branched isomers of hexane, mono-methylpentane and dimethylbutane respectively, have octane numbers that are significantly higher than that of n-hexane, with dimethylbutane having the highest research octane number (RON). Likewise, the singly branched isomer of pentane, 2-methylbutane, has a significantly higher RON than n-pentane. By increasing the proportion of these high octane isomers in the gasoline pool, satisfactory octane numbers can be achieved for gasoline without additional additives.
Two types of octane numbers are currently being used, the motor octane number (MON) determined using ASTM D2700-11 (“Standard Test Method for Motor Octane Number of Spark-Ignition Engine Fuel”) and the RON determined using ASTM D2699-11 (“Standard Test Method for Research Octane Number of Spark-Ignition Engine Fuel”). The two methods both employ the standard Cooperative Fuel Research (CFR) knock-test engine. Sometimes, the MON and RON are averaged, (MON+RON)/2, to obtain an octane number. Therefore, when referring to an octane number, it is essential to know which one is being discussed. In this disclosure, unless clearly stated otherwise, octane number will refer to the RON. For comparative purposes, the RON for isomers of pentane and hexane are listed in Table 1.
TABLE 1RONC5 paraffinsn-pentane622-methylbutane92C6 paraffinsn-hexane252-methylpentane743-methylpentane762,2-dimethylbutane942,3-dimethylbutane105 
Gasoline suitable for use as fuel in an automobile engine should have a RON of at least 80, e.g., at least 85, or at least 90. High performance engines generally require a fuel having a RON of about 100. Most gasoline blending streams have a RON generally ranging from 55 to 95, with the majority typically falling between 80 and 90. Obviously, it is desirable to maximize the amount of dimethylbutane in light paraffins of the gasoline pool in order to increase the overall RON.
Hydroisomerization is an important refining process whereby the RON of a refinery's gasoline pool can be increased by converting straight chain normal or singly branched light paraffins into their more branched isomers. The hydroisomerization reaction is controlled by thermodynamic equilibrium. At higher reaction temperatures, the equilibrium shifts towards the lower octane isomers (e.g., from dimethylbutanes via methylpentanes to n-hexane). Since the high octane components (e.g., 2,3-dimethylbutane with a RON=105) are the target products in this process, it is desirable to develop a more active catalyst to perform this reaction at a lower temperature.
There is a need for new and improved hydrocarbon hydroisomerization catalysts and processes that provide high selectivity for producing high octane isomers of light paraffins, wherein the catalysts are also highly active, environmentally benign, and readily regenerable.